How can i change backgrounds between frames in '2D animation' workspace?
A quick and dirty way would be to do this: 


Create your second background as another plane anywhere out of camera, then create a keyframe with i > Location for both planes on the last frame before you change shot. 
Go forward one frame (so to the first frame of your new shot), move the first background out of the way and put the second background in place, then create another location keyframe for both background planes.
Note that you should be in object mode to do these steps.


But, a much better way would be to use two separate scenes and cut between them using the VSE as explained in this tutorial at around 1h11m. It's worth watching the whole thing if you're just starting with grease pencil.